1. Stroke volume and cardiac output were measured using the Doppler ultrasound technique in 16 normal subjects immersed to the neck in water at 33°C, 35°C, 37°C and 39°C. A standard aortic diameter was assumed and results were expressed as percentage changes from pre-immersion resting values. 2. Cardiac output rose progressively at higher temperatures, increasing by 30% at 33°C and by 121% at 39°C. At thermoneutral temperatures (33°C and 35°C) this was achieved by an increase in stroke volume of 50% despite a significant decrease in heart rate. There was a further rise in stroke volume and pulse rate at higher temperatures and a mean tachycardia of 109 ± 4 beats/min was noted at 39°C. Calculated peripheral resistance reduced progressively with increasing temperature of immersion. 3. This non-invasive and simple technique may provide a non-exercise-related cardiovascular stress test to study cardiovascular responses in a variety of pathophysiological states.

1. The renal response to volume expansion produced by water immersion to the neck at 35°C was examined in eight young normotensive uncomplicated insulin-dependent diabetic subjects and in eight matched normal control subjects. 2. Both the diabetic and normal subjects manifested a renal response of natriuresis and kaliuresis on immersion, but the natriuretic response was reduced in the diabetic group. Thus the induced excretion of sodium over the 4 h of immersion was 40 ± 5 mmol (mean ± sem ) in the normal group compared with 22 ± 4 mmol in the diabetic group ( P < 0.02). 3. In the normal subjects creatinine clearance did not change during immersion compared with pre-immersion control values while in the diabetic group it rose from pre-immersion control values of 112 ± 11 ml/min to a mean value of 127 ± 11 ml/min during immersion ( P < 0.01). 4. The diabetic subjects thus excreted less sodium despite an increased filtered load during water immersion. Fractional excretion of sodium was significantly reduced in the diabetic subjects compared with the normal control subjects ( P < 0.05). 5. The suppression of plasma renin and aldosterone was similar in normal and diabetic groups. 6. Tubular sodium retention could be an early functional change in the diabetic kidney, and be implicated in the development of diabetic nephropathy.

1. Thermoneutral water immersion produces a physiological increase of thoracic blood volume, raises central venous pressure and increases urinary sodium excretion by a hitherto ill-understood mechanism. We have investigated whether this enhanced sodium excretion could be mediated by the recently discovered natriuretic factor, atrial natriuretic peptide (ANP). 2. During water immersion there was a highly significant ( P < 0.001) twofold increase of the mean plasma ANP concentration and a doubling of the mean urinary sodium excretion. Both were unchanged during the control experiments. 3. These results are consistent with the hypotheses (a) that ANP is released into plasma in response to central blood volume expansion and (b) that it functions as a natriuretic hormone in normal man under physiological conditions.

1. In 10 normal subjects water immersion to the neck at 35°C produced a highly significant natriuresis and diuresis. 2. Urinary prostaglandin E levels did not change significantly throughout immersion. 3. During the diuresis of water immersion a suppression of urinary antidiuretic hormone occurred.